This application is based upon and claims the benefit of priority of Japanese Patent Applications No. H.10-138846 filed on May 20, 1998 and No. H.11-9665 filed on Jan. 18, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine provided with a noble metal chip bonded on a center or ground electrode, in particular, an improvement in bonding strength of the chip made of an iridium (Ir) alloy.
2. Description of Related Art
A spark plug has generally a center electrode fitted through an insulator into a housing and a ground electrode integrated with the housing. The portion of the center electrode exposed out of the end of the insulator faces the ground electrode to form a spark gap within which a spark is discharged. To improve the life time and the performance of the spark plug, a noble metal chip is bonded on the center and/or ground electrode to constitute a spark discharge spot for the spark gap.
Conventionally, a platinum (Pt) alloy has been widely used as the material for the noble metal chip. However, the Pt alloy has, the deficiency that the consumption resistance thereof is not considered to be sufficient to meet the more demanding engine specifications for vehicles in the future. Therefore, the use of the iridium (Ir) alloy having a melting point higher than that of the Pt alloy has been recently studied and an iridium-rhodium (Irxe2x80x94Rh) alloy and the like have been proposed, as shown in P-A-9-7733.
The material of the center and/or ground electrode on which the chip is bonded is usually a nickel (Ni) base alloy. The difference between the linear expansion co-efficient of the Ni alloy and that of the Ir alloy is larger than the difference between that of the Ni alloy and that of the Pt alloy (for example, 90Pt-101r alloy, 80Pt-201r alloy and the like). Therefore, if a chip made of the Ir alloy is installed on the spark plug to be used in a high temperature combustion chamber, a great thermal stress due to the above mentioned larger difference in linear expansion co-efficients tends to be produced at the junction of the chip and the electrode according to the temperature change.
When a chip made of the Ir alloy is directly bonded on the electrode, laser beam welding is preferable to limit the possible separation of the chip and the electrode during its lifetime because the chip and the electrode may be sufficiently molten due to the high density of its energy. However, as the equipment cost and the fabrication cost of laser be am welding are relatively expensive, electric resistance welding may be desired in view of its inexpensive manufacturing cost, though the welding energy is lower, compared to laser beam welding.
It is well known, as described in JP-A-1-319284, when the chip and the electrode are bonded by resistance welding, to place a stress releasing layer having a linear expansion co-efficient intermediate between that of the chip and that of the electrode between the chip and the electrode in order to alleviate the thermal stress on the chip. The conventional spark plug for this purpose employs an Irxe2x80x94Ni alloy as the chip and a Ptxe2x80x94Ni alloy as the stress releasing layer.
According to the investigation of the present inventors, it was found effective to bond the Pt alloy on the electrode made of the Ni base alloy, but not always effective to bond the Ir alloy on the electrode in conventional ways. An endurance test result showed that using certain material combinations for the Ir alloy chip and/or the stress releasing layer results in a crack or a separation at the junction of the Ir alloy and the stress releasing layer and, as the worst case, the chip was left out from the stress releasing layer. The greater the weight percent of Ir contained in the Ir alloy for improving the consumption resistance of the chip, the more distinctively this problem has been observed. Further, when the diameter of the Ir alloy (for example, more than 1.5 mm) is relatively large, even if a plurality of the stress releasing layers are employed, because of the larger thermal stress produced, this problem can not be completely solved.
The present invention has been made in view of the above mentioned problem, and an object of the present invention is to provide a spark plug for internal combustion engines having the most suitable stress releasing layer. In particular, when the Ir alloy chip is bonded through the stress releasing layer on the Ni base alloy center and/or ground electrode by the resistance welding, it is preferable that the value of Young""s modulus of the stress releasing layer is less than those of the Ir alloy chip and the Ni base alloy electrode and, further, the value of linear expansion co-efficient of the stress releasing layer is intermediate between those of the Ir alloy chip and the Ni base alloy electrode. The stress releasing layer having the above mentioned value of Young""s modulus can effectively absorb or alleviate the thermal stress at the junction of the chip and stress releasing layer, thus, improving the bonding strength of the chip bonded by the resistance welding.
Even if the Ir alloy or the Ir alloy having at least one of material such as rhodium (Rh), platinum (Pt), ruthenium (Ru), palladium (Pd) and tungsten (W) contains more than 50 weight percent (Wt %) of Ir, the above mentioned stress releasing layer will serve to prevent the separation or the crack at the junction.
More particularly, it is preferable to use a stress releasing layer, whose Young""s modulus falls within 5xc3x97104 Mpa and 15xc3x97104 Mpa at a temperature of 900xc2x0 C., under which the spark plug is generally exposed in the engine combustion chamber at a full load operation of the engine ( for example, at an engine revolution of 6000 rpm). The lower limit of Young""s modulus, 5xc3x97104 Mpa, was determined from the standpoint that, when the Young""s modulus of the stress releasing layer is less than the above lower limit, there is a fear of producing a crack, not at the junction portion, but on the stress releasing layer itself because the material is too soft as its nature. Further, it is preferable that the linear expansion co-efficient of the stress releasing layer falls within 10xc3x9710xe2x88x926 (/xc2x0 C.) and 11xc3x9710xe2x88x926 (/xc2x0 C.).
To achieve the above object, an alloy containing Pt or a Ptxe2x80x94Irxe2x80x94Ni alloy, more specifically, an alloy containing 65 to 89 Wt % of Pt, 10 to 30 Wt % of Ir and 1 to 5 Wt % of Ni, can be used as material of the stress releasing layer.
As to the thickness of the stress releasing layer, more than 0.2 mm is found to be preferable to obtain a reliable bonding strength according to the experimental test result of the present inventors. If the thickness of the stress releasing layer is less than 0.2 mm, a crack tends to be produced on the stress releasing layer itself. On the other hand, if the thickness of the stress releasing layer exceeds 0.6 mm, the bonding strength is saturated. Therefore, the upper limit of the thickness of the stress releasing layer is preferably 0.6 mm in view of material cost savings.
Furthermore, it will be effective, in particular in the case of a relatively large diameter of the chip, to employ two stress releasing layers for alleviating the thermal stress step by step. In addition to a first stress releasing layer as mentioned above, there is provided, between the first stress releasing layer and the electrode, a second stress releasing layer having a linear expansion co-efficient intermediate between those of the first stress releasing layer and the electrode. For this purpose, a Ptxe2x80x94Ir alloy may be used as the first stress releasing layer and a Ptxe2x80x94Ni alloy as the second stress releasing layer.
It is another object to provide a spark plug for internal combustion engine having an Ir alloy chip to be bonded through a stress releasing layer on a Ni base alloy electrode, in which the junction of the chip and the stress releasing layer is constituted by a curved surface. As described in FIG. 13, the conventional junction has a plain surface portion J3 and an edge portion J4 formed by a part of the disk or column type chip J1 buried into the stress releasing layer J2 when the chip J1 is bonded by the resistance welding on the stress releasing layer J4 whose diameter is larger than that of the chip J1. The bonding strength of the edge portion J4 is inherently weak as the nature of the resistance welding and, further, the thermal stress is focussed on the edge portion J4 as it is affected in the directions shown in arrows in FIG. 13. Therefor, it may be considered that the separation of the chip J1 from the stress releasing layer J2 tends to occur from the edge portion J4.
On the other hand, the thermal stress is dispersed uniformly on whole area of the junction in case of the curved junction surface without the edge portion. The experimental result of the present inventors has clearly proved that there was a big difference between the plain surface with the edge portion and the uniformly curved surface with respect to the separation percentage of the chip and the stress releasing layer.
The further object of the present invention is to provide a spark plug having a plurality of chips to be bonded on a single stress releasing layer. As the size of the chip is larger, the thermal stress is more heavily affected on the chip. Therefor, if the chip can be constituted by a plurality of smaller chips and the respective smaller chips are bonded on a single stress releasing layer, the thermal stress impact on each of the chips may be alleviated and the bonding strength of the chip as a whole can be improved.
It is desired, further, that the diameter of each disk type chip be less than 1.5 mm before bonding on the stress releasing layer. Furthermore, it will be more effective for improving the bonding strength to have a combination where the chip is constituted by a plurality of small chips and the junction of each chip and the stress releasing layer is shaped as a curved surface. In this case, the preferable range of the diameter of the chip is between 2.0 mm and 0.1 mm.
Finally, as a method of forming the curved surface junction of the chip and the stress releasing surface, it is preferable that a ball type chip made of an Ir alloy containing more than 50 Wt % of Ir is bonded through the stress releasing layer on the electrode by resistance welding.